The entrepreneur who owns the rights to Daraprim, a medicine that treats dangerous parasitic infections in children and adults alike, is under considerable fire after he raised the price of one pill from $13.50 to $750, overnight.

The Infectious Diseases Society of America and the HIV Medicine Association sent a letter to Turing Pharmaceuticals, arguing the price hike was way too steep, the Guardian reported.

“[It’s] unjustifiable for the medically-vulnerable patient population” and “unsustainable for the health care system,” the letter stated, the New York Times reported.

And Judith Aberg, with the Icahn School of Medicine at Mount Sinai hospital in New York, asked in the Guardian: “What is it that they are doing differently that has led to this dramatic increase?”

Turning is owned by Martin Shkreli, a former hedge fund manager. He acquired Daraprim, which is used to treat toxoplasmosis, an uncommon but fatal infection in babies and adults with compromised immune systems, in August and shortly after, raised the price.